The common carrier direct distance dialing (DDD) network continues to provide most of the telephonic communication capability for both individual and business customers. As described in the article entitled "Common Channel Interoffice Signaling" (CCIS) in the February 1978 issue of The Bell System Technical Journal, signaling for interoffice call set-up and take-down in the DDD network can be delegated to a common signaling link. Separation of the signaling from the voice channel and flexibility in the assignment of message formats on the CCIS network permits evolution and changes to the DDD network to be made at moderate cost. Also, CCIS provides faster interoffice call set-up and take-down.
In the CCIS network, a central office is provided with an access link to a signal transfer point (STP) and the STP in one region is connected to other STPs in each neighboring region. The STPs are provided in the CCIS network in order to permit non-associated signaling, i.e., the exchange of signaling information between offices not directly connected by a CCIS link. In addition to call set-up and take-down, information for network management such as alternate routing priorities, etc. may also be carried by the CCIS trunks. The use of common channel interoffice signaling reduces the holding time of the associated toll trunks, reduces post dialing delays and permits the implementation of an ever-growing number of new telephonic services.
Many large business enterprises today have a number of private branch exchanges that rival the common carrier public switched (DDD) network in the extent of geographical dispersion. Also, some branch exchanges challenge the size and complexity of central offices in terms of the number of features, trunks and stations served.
As the communication needs of a business organization expand, it becomes desirable to transcend the specific switching limitations of each individual PBX so that users served by a group of physically separate PBXs may be given the same calling privileges as if they were served by one large PBX. It is advantageous not only to connect together geographically dispersed PBXs without resorting to the common carrier DDD toll network but also, to provide a uniform numbering plan for all locations, network cost control features, such as automatic alternate routing and off-hook and ringback queuing to accomplish an efficient and least cost routing. A corporate PBX network achieving some of these goals is described in the articles entitled "Expanding the Role of Private Switching Systems"; Bell Labs Record, Vol. 57, No. 9, October 1979, pp. 242-248, and in "Development of Electronic Tandem Service (ETS) Features for the DIMENSION PBX", Proc. IEEE Computer Society, Nov. 6-8, 1979, pp. 86-93.
While the electronic tandem PBX network certainly represents an improvement in the ability to link together individual PBX's, it is not feasible to transport all types of feature-related or call-related information by using tandem tie-trunks primarily due to signaling limitations and data transfer rates. Transmitting such information over the trunks would impose a significant workload on the common control processors of the PBX nodes.
U.S. Pat. No. 4,313,036 issued to M. D. Jabara et al on Jan. 26, 1982, proposes a scheme that provides both a voice network and a packet network between computerized branch exchanges for the serving of interPBX calls. A first PBX interrogates a second PBX over the packet network to determine the busy/idle status of a called station at the second PBX. The second PBX returns the busy/idle status of the called station to the first PBX via the packet network and if that station is idle, it reserves that called station at the second PBX for serving the call when it is extended by the first PBX. The originating PBX, in essence, "looks ahead" to determine if the called station is available in the second PBX and provides a connection via the voice network following such a determination.
It might appear that Jabara et al's arrangement provides a satisfactory solution with respect to linkages between PBXs. However, this arrangement does not provide service or feature transparency. In Jabara et al, no information is exchanged following a connection set-up between co-located or geographically dispersed PBXs. This precludes any implementation of the features or services found in a single PBX system. Therefore, the customer having dispersed PBXs as in Jabara et al, would not perceive the plurality of PBXs operating as a single entity when a particular feature or service is required.
It might at first blush appear that the provision of interPBX, CCIS-like links could contribute a similar flexibility of service offerings to a corporate PBX network as are achieved between central offices in the common carrier DDD network. This approach would require a dedicated link between each control processor located in each communicating pair of PBXs. In the common carrier networks, the dedicated high cost CCIS links can be economically justified for public switching because of the heavy volume of offered traffic. In a private PBX network, however, the volume of traffic offered to a PBX CCIS-like link would likely be much lower than that in the public network.
The presence of STPs in the CCIS network reduces the number of direct links required between the COs. The inclusion of STP-type switch units into a group of co-located or geographically dispersed PBXs might, perhaps, appear to address the issue of cost since the number of links between PBXs would be reduced. However, the STPs are expensive switching units that merely act as relay points for the transfer of data. Also, the STPs provide no processing capability with regard to receiving the call stimulus, formatting data with respect to call stimulus, determining the appropriate destination and other such data processing operations necessary for establishing an interPBX call connection in a transparent manner.
A perplexing problem, then, is how economically to implement CCIS-like links in a private PBX network without incurring the expense of providing direct links between all of the PBXs or the expense of providing a network of STP facilities. The use of data links between PBX switching nodes would permit the interchange not only of the same kind of information exchanged over CCIS links between central offices (trunk signaling, traveling class marks, called station supervision, etc.) but also of any additional information that might be of especial utility to the particular corporate network PBX customer, for example, feature-related, call-related, station-related and location-related information.
For example, the PBX attendant at a given PBX node can usually quite easily be provided with a lamp display to indicate the condition of any group of stations or trunks directly terminating in the given PBX where the attendant is located. However, it has heretofore not been economically attractive to provide facilities that would permit the condition of stations or trunks of another PBX in the network to be displayed.
Where a PBX customer has a greater number of telephone stations in a given locality than can be served from a single PBX, it is sometimes desirable to serve such a customer by a plurality of PBXs which together, have the required station capacity. It is then desirable that the plurality of PBXs appear as a single PBX to the users. In other words, the plurality of PBXs are transparent to the users and appear as a single PBX. Accordingly, transparency may be defined as the provision of a service feature in such a manner that the user can perceive no differences occasioned by the need to physically distribute circuits or control. Transparency is also desirable for the plurality of PBXs comprising a multilocation PBX network.
It is accordingly a goal of the present invention to provide full feature transparency among geographically dispersed or co-located PBXs, including features requiring call set-up or completion within specified time limits.